A wide variety of portable communication products (e.g., cellular phones, PDAs (personal digital assistants), Internet phones, etc.) are commercially available in an even faster pace in recent years as electronics industry advances. Moreover, such portable communication products are provided with many advanced features. Each group of the portable communication products is able to operate in predetermined frequencies due to its specific functions and wireless communication system involved. Currently, dominant wireless communication systems are GSM (Global System for Mobile Communications) (i.e., so-called Pan-European digital mobile phone system) and CDMA (Code Division Multiple Access). For GSM, its frequency bands are 850 MHz, 900 MHz, 1800 MHz, and 1900 MHz. For CDMA, its frequency bands are 800 MHz and 1900 MHz.
Different wireless communication systems are currently employed throughout the world. Thus, a cellular phone user may have to carry another type of cellular phone if he/she travels to another country due to different specifications. For example, GSM is employed in USA and its frequency bands are 850 MHz and 1900 MHz, GSM is employed in European countries and its frequency bands are 900 MHz and 1800 MHz, CDMA is employed in Korea and its frequency bands are 800 MHz and 1900 MHz, GSM is employed in Australia and its frequency bands are 900 MHz and 1800 MHz, and GSM is employed in South East Asian countries and its frequency bands are 900 MHz and 1800 MHz respectively.
In view of the above discussion, cellular phones of single operating frequency are somewhat outdated. Currently, a number of major portable communication product manufacturers (e.g., Motorola, Nokia, Sony Ericson, etc.) have developed dual-band or triple-band portable communication products (e.g., triple-band cellular phones) for solving the above drawback. For a triple-band cellular phone, it is capable of operating in bands including 900 MHz, 1800 MHz, 1900 MHz, and 2.4 GHz and is adapted to operate in GSM, CDMA, or wireless Internet. Alternatively, it can be used as a PHS (Personal Handy-phone System) or Bluetooth cellular phone, or can operate in GPS (Global Positioning System) or other wireless networks. By using the triple-band cellular phone, a user does not have to change his/her cellular phone or any carried portable communication product when the user travels from one country to another country.
For a multi-mode portable communication product employing dual-band or triple-band for operating in bands including 1800 MHz, 1900 MHz, and 2.4 GHz, the most important component thereof is antenna. For example, U.S. Pat. No. 6,112,102 discloses a multi-band non-uniform helical antenna as shown in FIG. 1. The helical antenna 1 comprises a first coil section 10 and a second coil section 20 having a spiral angle, a coil diameter, a length, turns, and a pitch all different from that of the first coil section 10. Thus, each of the first and second coil sections 10 and 20 is formed as a helical antenna operating in dual-band or triple-band mode. In a case of the helical antenna 1 installed in a multi-mode portable communication product employing dual-band or triple-band, it is possible of tuning the helical antenna 1 to a plurality of resonant frequencies by changing parameters including spiral angle, coil diameter, length, turns, and pitch of the helical antenna 1. As an end, the purpose of operating in different frequency bands is achieved.
Taiwanese Patent No. 549,621 discloses a multi-band helical antenna for communication equipment as improvement of U.S. Pat. No. 6,112,102 entitled “Multi-band Non-uniform Helical Antenna”. As discussed in background of the Taiwanese Patent No. 549,621, predetermined parameters such as spiral angle and pitch of the antenna in U.S. Pat. No. 6,112,102 may be changed due to carelessness in the manufacturing process. Thus, in often times the antenna in U.S. Pat. No. 6,112,102 either may not be able to operate normally in receiving or transmitting signals or cannot achieve the performance of a desired multi-band antenna. Therefore, a positioning member 2 is provided in a helical antenna 1 as disclosed in Taiwanese Patent No. 549,621 in which the positioning member 2 is firmly retained in the antenna 1 and thus the antenna 1 is able to operate in a third frequency band as shown in FIG. 2.
Above Taiwanese Patent No. 549,621 entitled “Multi-band Helical Antenna for Communication Equipment” employs the positioning member 2 as means for operating in a third frequency band in which the positioning member 2 comprises a metal patch 20 on its surface for cooperating with the helical coils. That is, the purpose of tuning the antenna 1 to a plurality of resonant frequencies by the positioning member 2 depends on parameters of the helical antenna 1. This means that location and shape of the metal patch 20 are constrained by the antenna 1. That is, the provision of the positioning member 2 is trouble-prone and unreliable in use. For example, a plurality of recesses 22 on both sides of the positioning member 2 must be conformed to coil pitches. Incorrect location of the recesses 22 will cause incompatibility of the antenna 1 and the positioning member 2. As a result, it is impossible of tuning the antenna 1 to a plurality of resonant frequencies during operation. Moreover, shapes of the recesses 22 must be snugly fitted between two adjacent coils. It is impossible of fastening the antenna 1 in the recesses 22 if the recess 22 is sufficiently larger than coil pitch. To the contrary, it is also impossible of mounting coils of the antenna 1 in the recesses 22 if the recess 22 is much smaller than coil pitch. Thus, the need for improvement still exists.